Among petroleum products, jet fuels, diesel oils, lubricant oils, and the like are the products whose fluidity at a low temperature is important. For this reason, desirably, in a base oil used for these products, wax components such as normal paraffins that cause the fluidity at a low temperature to be reduced are completely or partially removed, or converted to a component other than the wax components. Moreover, recently, hydrocarbon oils obtained by a Fischer-Tropsch synthesis (hereinafter, written as the “FT synthesis” in some cases) have received attention as a feedstock for producing fuel oils and lubricant oils because the content of substances of concern such as sulfur is small; however, a large amount of the wax components are contained also in the hydrocarbon oils.
As a dewaxing technique for removing a wax component from a hydrocarbon oil, a method of extracting a wax component by a solvent such as liquefied propane and methyl ethyl ketone is known. However, in the method, there are problems such as increase in the scale of the apparatus, expensive operating cost, limitation of the kind of an applicable feedstock, and limitation of the yield of the product by the kind of the feedstock.
On the other hand, as a dewaxing technique for converting a wax component in a hydrocarbon oil into a non-wax component, a contact dewaxing technique is known in which a hydrocarbon oil is contacted with a hydrogenation isomerization dewaxing catalyst having both a hydrogenation-dehydrogenation ability and an isomerization ability in the presence of molecular hydrogen to isomerize normal paraffins in the hydrocarbon oil to isoparaffins, for example.
The contact dewaxing is effective as a method for improving the fluidity at a low temperature of the hydrocarbon oil, but a conversion rate of the normal paraffins needs to be sufficiently high in order to obtain a fraction suitable for a base oil for a lubricant oil. However, a hydrogenation isomerization catalyst used in the contact dewaxing has both an isomerization ability and an ability to crack hydrocarbons; for this reason, cracking of the hydrocarbon oil and production of lighter products progress as the conversion rate of the normal paraffins is increased, and it is difficult to obtain a desired fraction with high efficiency. Particularly, in production of a high quality base oil for a lubricant oil of which a high viscosity index and a low pour point are demanded, it is very difficult to obtain a target fraction economically; for this reason, synthetic base oils such as poly-alpha-olefins have been used often in the field.
From such a circumstance, in the field of production of the lubricant base oil, a dewaxing technique for efficiently obtaining a desired isoparaffin fraction from a hydrocarbon oil containing a wax component has been demanded.
So far, an attempt to improve the isomerization selectivity of the hydrogenation isomerization catalyst used in the contact dewaxing has been made. For example, Patent Literature 1 below discloses a process in which a linear or slightly branched hydrocarbon raw material having not less than 10 carbon atoms is contacted under an isomerization condition with a catalyst comprising a molecular sieve such as ZSM-22, ZSM-23, ZSM-48 containing a metal of Group VIII to Group X in the periodic table and having one-dimensional pores of a middle size in which the size of a crystallite does not exceed approximately 0.5μ, thereby to produce a dewaxed lubricant oil (Patent Literature 1).